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The sacrum (: sacra or sacrums Oxford Dictionaries and Webster's New College Dictionary (2010) admit the plural sacrums alongside sacra; The American Heritage Dictionary, Collins Dictionary and Webster's Revised Unabridged Dictionary (1913) give sacra as the only plural.), in human body, is a triangular bone at the base of the vertebral column that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30.
The sacrum situates at the upper, back part of the pelvic cavity, between the two wings of the pelvis. It forms joints with four other bones. The two projections at the sides of the sacrum are called the alae (wings), and articulate with the ilium at the L-shaped . The upper part of the sacrum connects with the last lumbar vertebrae (L5), and its lower part with the coccyx (tailbone) via the sacral and coccygeal cornua.
The sacrum has three different surfaces which are shaped to accommodate surrounding pelvic structures. Overall, it is (curved upon itself). The base of the sacrum, the broadest and uppermost part, is tilted forward as the sacral promontory internally. The central part is curved outward toward the posterior, allowing greater room for the pelvic cavity.
In all other vertebrates, the pelvic vertebrae undergo a similar developmental process to form a sacrum in the adult, even while the bony tail (caudal) vertebrae remain unfused. The number of sacral vertebrae varies slightly. For instance, the S1S5 vertebrae of a horse will fuse, the S1S3 of a dog will fuse, and four pelvic vertebrae of a rat will fuse between the lumbar and the caudal vertebrae of its tail.
The Stegosaurus dinosaur had a greatly enlarged neural canal in the sacrum, characterized as a "posterior brain case".
In the middle of the base is a large oval articular surface, the upper surface of the body of the first sacral vertebra, which is connected with the under surface of the body of the last lumbar vertebra by an intervertebral fibrocartilage. Behind this is the large triangular orifice of the sacral canal, which is completed by the lamina and spinous process of the first sacral vertebra. The superior articular processes project from it on either side; they are oval, concave, directed backward and medialward, like the superior articular processes of a lumbar vertebra. They are attached to the body of the first sacral vertebra and to each ala, by short thick pedicles; on the upper surface of each pedicle is a vertebral notch, which forms the lower part of the foramen between the last lumbar and first sacral vertebrae.
The apex is directed downward and presents an oval facet for articulation with the coccyx. The sacral canal as a continuation of the vertebral canal runs throughout the greater part of the sacrum. The sacral angle is the angle formed by the true conjugate with the two pieces of sacrum. Normally, it is greater than 60 degrees. A sacral angle of lesser degree suggests funneling of the pelvis.
The dorsal surface of the sacrum is convex and narrower than the pelvic surface. In the middle line is the median sacral crest, surmounted by three or four tubercles—the rudimentary of the upper three or four sacral vertebrae. On either side of the median sacral crest is a shallow sacral groove, which gives origin to the multifidus muscle. The floor of the groove is formed by the united vertebral lamina of the corresponding vertebrae. The laminae of the fifth sacral vertebra, and sometimes those of the fourth, do not meet at the back, resulting in a fissure known as the sacral hiatus in the posterior wall of the sacral canal. The sacral canal is a continuation of the spinal canal and runs throughout the greater part of the sacrum. Above the sacral hiatus, it is triangular in form. The canal lodges the sacral nerves, via the anterior and posterior sacral foramina.
On the lateral aspect of the sacral groove is a linear series of tubercles produced by the fusion of the articular processes which together form the indistinct medial sacral crest. The articular processes of the first sacral vertebra are large and oval-shaped. Their facets are concave from side to side, face to the back and middle, and articulate with the facets on the inferior processes of the fifth lumbar vertebra.
The tubercles of the inferior articular processes of the fifth sacral vertebra, known as the sacral cornua, are projected downward and are connected to the cornua of the coccyx. At the side of the articular processes are the four posterior sacral foramina; they are smaller in size and less regular in form than those at the front, and transmit the posterior divisions of the sacral nerves. On the side of the posterior sacral foramina is a series of tubercles, the transverse processes of the sacral vertebrae, and these form the lateral sacral crest. The transverse tubercles of the first sacral vertebra are large and very distinct; they, together with the transverse tubercles of the second vertebra, give attachment to the horizontal parts of the posterior sacroiliac ligaments; those of the third vertebra give attachment to the oblique fasciculi of the posterior sacroiliac ligaments; and those of the fourth and fifth to the sacrotuberous ligaments.
The lateral surface of the sacrum is broad above, but narrows into a thin edge below. The upper half presents in front an ear-shaped surface, the auricular surface, covered with cartilage in the immature state, for articulation with the ilium. Behind it is a rough surface, the sacral tuberosity, on which are three deep and uneven impressions, for the attachment of the posterior sacroiliac ligament. The lower half is thin, and ends in a projection called the inferior lateral angle. Medial to this angle is a notch, which is converted into a foramen by the transverse process of the first piece of the coccyx, and this transmits the anterior division of the fifth sacral nerve. The thin lower half of the lateral surface gives attachment to the sacrotuberous and sacrospinous ligaments, to some fibers of the gluteus maximus at the back and to the Coccygeus muscle in the front.
Rotation of the sacrum superiorly and anteriorly whilst the coccyx moves posteriorly relative to the ilium is sometimes called "nutation" (from the Latin term which means "nodding") and the reverse, postero-inferior motion of the sacrum relative to the ilium whilst the coccyx moves anteriorly, "counter-nutation". In upright vertebrates, the sacrum is capable of slight independent movement along the sagittal plane. On bending backward the top (base) of the sacrum moves forward relative to the ilium; on bending forward the top moves back.Maitland, J (2001). Spinal Manipulation Made Simple. Berkeley: North Atlantic Books, p. 72.
The sacrum refers to all of the parts combined. Its parts are called sacral vertebrae when referred individually.
Another congenital disorder is that of caudal regression syndrome also known as sacral agenesis. This is characterised by an abnormal underdevelopment in the embryo (occurring by the seventh week) of the lower spine. Sometimes part of the coccyx is absent, or the lower vertebrae can be absent, or on occasion a small part of the spine is missing with no outward sign.
The origin of Galen's term is unclear. Supposedly the sacrum was the part of an animal offered in sacrifice (since the sacrum is the seat of the organs of procreation). Others attribute the adjective ἱερόν to the ancient belief that this specific bone would be indestructible. As the Greek adjective ἱερός may also mean "strong", it has also been suggested that os sacrum is a mistranslation of a term intended to mean "the strong bone". This is supported by the alternative Greek name μέγας σπόνδυλος by the Greeks, translating to "large vertebra", translated into Latin as vertebra magna.
In Classical Greek the bone was known as κλόνις (Latinized clonis); this term is cognate to Latin clunis "buttock", Sanskrit "haunch" and Lithuanian šlaunis "hip, thigh".used by Antimachus; see The Latin word is found in the alternative Latin name of the sacrum, ossa clunium, as it were "bones of the buttocks". Due to the fact that the os sacrum is broad and thick at its upper end, the sacrum is alternatively called os latum, "broad bone".
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